1. Field of the Invention
The present invention relates to communications; more specifically, wireless communications.
2. Description of the Related Art
Wireless communications involve creating a voice or data communication channel between a mobile communication station and a base station. Setting up the communication channel typically involves the mobile station transmitting a known sequence on an access channel that is monitored by the base station. The base station detects the known sequence and uses it for functions such as estimating a timing difference between the mobile station and base station.
The signal transmitted by the mobile station to the base station over an access channel typically includes a known sequence based on one of M possible signature sequences comprising S symbols. In one such system, M=16 different signature sequences are available, where each signature sequence comprises S=16 symbols. UMTS W-CDMA uses length 16 Walsh-Hadamard sequences as signature sequences. These sequences are well known in the art and are described on pages 15-16 of 3GPP TSG RAN “Spreading and Modulation (FDD),” TS25.213 V3.2.b. Once one of the 16 symbol signature sequences is selected, it is used to generate a sequence that is transmitted to the base station. FIG. 1 illustrates how the transmit sequence is generated from a 16 symbol signature sequence. Sequence 10 represents a 16 symbol signature sequence with symbol periods 12, where each symbol is +1 or −1. Each of the 16 symbol periods is divided into C chip or sample periods 14, in this example C=256. As a result, the signature sequence comprises a total of K chip or sample periods, where K=4,096 (S=16 symbol periods×C=256 chip periods per symbol period). The signature sequence is used to generate interleaved sequence 18. The interleaved sequence comprises 256 (K/C) repeating periods 20, each with 16 (S) chip periods 22 for a total of 4,096 (K) chip periods. The interleaved sequence is created by using the symbol values in the first chip period of symbol periods 0 through 15 of signature sequence 10 to populate the first 16 chip periods of repeating period 0 of interleaved sequence 18. The chip periods of repeating period 1 of interleaved sequence 18 are populated using the symbol values in the second chip periods of each of the 16 symbol periods of signature sequence 10. Similarly, the chip periods of repeating period 2 of interleaved sequence 18 are populated using the symbol values in the third chip periods of symbol periods 0 through 15 of signature sequence 10. This process continues until the 16 chip periods of the last repeating period (repeating period 255) are populated using the symbol values in the last chip period of each of the 16 symbol periods of signature sequence 10. As a result, interleaved sequence 18 consists of 256 repeating periods each containing 16 chip periods. Each of the repeating periods contains 16 chip periods having values equal to the value of one chip period from each symbol period of signature sequence 10. Therefore, a sample of symbol periods 0 through 15 of signature sequence 10 is contained in chips 0 through 15, respectively, of each repeating period of interleaved sequence 18.
The final step in generating a known sequence that is transmitted from the mobile station to the base station involves performing a chip period by chip period multiplication of interleaved sequence 18 with a 4,096 (K) chip period binary sequence 24. Binary sequence 24 is known and assigned to the particular base station with which the mobile will communicate. The result of the chip period by chip period multiplication is transmit sequence 26 which is then transmitted by the mobile to the base station.
The set of possible transmit sequences 26 is known by the base station that will receive the mobile transmission. The available signature sequences, the binary sequence and the interleave pattern are known, and as a result, the set of possible transmit sequences 26 is also known for each of the available signature sequences.
FIG. 2 illustrates a multiple signal detector used by the base station to identify and detect known sequences transmitted by a mobile station and received at the base station. Shift register 30 receives samples of the received sequence. Shift register 30 has 4,096 (K) locations in order to provide for 4,096 samples which correspond to the 4,096 chip periods that compose the received sequence. In order to account for the interleaving that was used to create the received sequence, a deinterleaving process is carried out while providing samples from shift register 30 to correlators 32, 34 and 36. It should be noted that the first chip period of each 16 chip long repeating period is provided to correlator 32. Similarly, the second chip period of each 16 chip long repeating period is provided to correlator 34. This process continues for a total of 16 correlators where the 16th correlator or correlator 36 receives the last chip of each 16 chip long repeating period. This deinterleaving process provides each correlator with 256 chip period samples of a symbol period. Each of the correlators is provided with coefficients representative of a sequence of values associated with the 256 chip period values that represent a symbol. It should be noted that the sequence of coefficients provided to the correlator take into account the chip period by chip period multiplication that occurred between interleaved sequence 18 and binary sequence 24. The output provided by each correlator indicates how well the 256 chip period values from a symbol period match the sequence of chip period values that are expected for a +1 or −1 symbol. As a result, Fast Hadamard Transform (FHT) 40 receives an input from each of the 16 correlators where each input represents how well the 256 chip period values being examined by the correlator correspond to a symbol and whether that correspondence is to a +1 or −1 symbol.
FHT is well known in the art and are discussed in references such as “Fast transforms: algorithms, analysis, applications,” pages 301-329, by D. Elliot and K. Rao, Academic Press, Orlando, Fla., 1982. FHT 40 is provided with coefficients that are used to identify which of 16 possible signature sequences are being received based on the outputs provided by the correlators. The FHT provides 16 outputs each corresponding to one of the possible signature sequences, where the magnitude of the output indicates how well the samples in shift register 30 matches each sequence. FHT 40 outputs are each provided to absolute value generator 42 which takes the absolute value or the square of the absolute value of the output for each FHT output. Each of the outputs of absolute value generator 42 is provided to thresholder 44 which compares the value from absolute value generator 42 with a predetermined threshold. When the value exceeds the threshold, a detection is declared and the received sequence is identified as corresponding to a particular signature sequence by the FHT output that produced the threshold-exceeding signal.
It should be noted that the base station attempts to detect the sequence over a period of time referred to as a search window. A search window is typically N times the sampling period of the received sequence. Once shift register 30 is filled with an initial set of samples, it shifts in new samples and shifts out older samples N-1 times. This results in N attempts to detect the expected sequence over a search window that is equal to N times the time period between samples provided to shift register 30. A detected sequence's position in the search window is determined by the number of shifts made by shift register 30 when one of the FHT's outputs corresponding to a signature pattern to be detected exceeds a threshold. The detected sequence's position in the search window is a measure of the round trip delay between the mobile station and the base station.
When the mobile station is in a fast moving motor vehicle or train, the signal supplied to the shift register is subjected to fast fading and frequency offsets. As a result, the sequence received by the shift register is partially corrupted and produces a low FHT output. As a result, the FHT outputs that are compared with a threshold do not exceed the threshold and thereby result in a failure to detect or identify a received signature sequence.